Electrostatic apparatus for paper detacking



m Dec. 12, 1967 A.T. MAPKGHERMALAN! y f ELECTROSTATIC APPARATUS FOR PAPER DETACKING Filed Oct. 11, 1965 INVENTOR ARJAN 'I'. MANGHIRMALANI ATTORNEYS United States Patent "cc 3,357,400 ELECTROSTATIC APPARATUS FQR PAPER DETAQKING Arjan T. Maughirmalani, Pittsford, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 11, 1966, Ser. No. 585,826 5 Claims. (Cl. 118-637) This invention relates to xerography and in particular to apparatus for stripping copy with transferred toner images from a xerographic surface.

In the practice of xerography, as described for example in US. Patent No. 2,297,691, to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material afiixed to a conductive backing is used to support latent electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as a powder. The powder is held in image areas by the electrostatic charge on the layer. Where the charge is greatest, the greatest amount of material is deposited; and where the charge is least, little or no material is deposited. Thus, a powdered image is produced in conformity with a light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably affixed thereto to form a permanent print.

The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred herein to as toner and a coarse granular material called carrier. The carrier is coated with a material removed in the triboelectric series from the toner so that a charge is generated between the powder and the granular carrier upon mutual interaction. Such charge causes the powder to adhere to the carrier. The carrier, besides providing a charge to the toner, permits mechanical control so that the toner can readily be brought into contact with the exposed xerographic surface for the development of the surface. The powder particles are attracted to the electrostatic image from the granular material to produce a visible powdered image on the xerographic surface.

When xerography is practiced on a cylindrically shaped endless xerographic surface, as a drum described for example in US. Patent 3,062,536 to J. Rutkus, Jr., et al., or US. patent application Serial No. 400,363 to Osborne et al., copending herewith, the final copy paper is caused to move in synchronous contact with the drum during a portion of the rotation of the drum. During this time, a potential opposite from the polarity on the toner is applied to the side of the paper remote from the drum to electrostatically attract the toner image from the drum to the copy paper. A pad of air is then employed to separate the image bearing paper from the drum. The toner image is then fused to the paper for the production of the final xerographic copy.

During the transfer of the toner image from the drum to the paper, the transfer corotron, which is of a positive bias when negatively charged toner is applied, deposits a uniform positive charge across the copy paper. The

copy paper which is an insulator, then induces a nega- 3,357,400 Patented Dec. 12, 1967 tive charge in the non-image areas of the xerographic drum surface which thus creates an electrostatic bond between the paper and drum.

To separate the paper from the drum, the electrostatic bond therebetween must first be overcome. When an air puffer is used to achieve this separation, a relatively high pressure of air must be employed to overcome the attraction of the paper for the drum. However, when high air pressures are employed, there is a tendency of such air pressure to agitate the unfused toner image on the paper and disrupt the image configuration of the toner. This exhibits itself as puffer smears on the final copy. This blowing of toner powder also results in -mechanical dirt problems throughout the functioning elements of the system. Furthermore, when a copy is being separated from the xerographic surface the high pressure of air can repel the toner bearing paper against the output conveyor with a toner-jarring force when the electrostatic bond is small.

The present invention is directed to apparatus for reducing the electrostatic bond between the paper and xerographic surface so that the puffer pressure required to separate the copy paper from the xerographic surface is, minimized.

It is, therefore, an object of the instant invention to strip image bearing xerographic copy from a xerographic surface.

It is another object of the invention to reduce the air pressure required to pneumatically strip xerographic copy from a xerographic surface.

It is another object of the invention to neutralize the electrostatic bond between xerographic copy and a Xerographic surface prior to their separation regardless of the amount of image areas.

It is another object of the invention to electrostatically minimize the bond between xerographic copy and a xerographic surface so as to facilitate their physical separa tion.

It is a further object of the invention to reduce the amount of toner smears caused by puffing developed copy from a xerographic surface.

It is still a further object of the invention to reduce the amount of toner dust blown throughout xerographic systems during puff type stripping.

These and other objects of the invention are achieved by neutralizing the electrostatic bond between Xerographiccopy and xerographic surfaces after an electrostatic transfer. This is achieved by the use of AC detacking corona emissions. According to the invention, however, the xerographic image bearing surface must first be exposed to AC pro-transfer corona emissions so that the charge differential between the image and non-image areas of the image bearing xerographic surface is of a decreased range. This decreased range of potentials permits the detacking corotron to emit a substantially uniform rate of neutralizing emissions regardless of whether line copy drum charges or extended solid area drum charges are presented to the detacking corotron. This is of special importance when the machine, in which the invention is incorporated, is modified to allow improved solid area development capabilities.

For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawing wherein the figure is a schematic diagram of an automatic xerographic reproducing machine utilizing the principles of the instant invention.

Re ferring now to the figure, there is shown an embodiment of the subject invention in a suitable environment such as an automatic xerographic reproducing machine, the machine has a xerographic surface formed by a photoconductive layer on a conductive backing, the surface is formed in the shape of a drum, which is mounted on a shaft journaled in the frame of the machine to rotate in a direction as indicated by the arrow. This causes the drum surface to move sequentially past a plurality of xerographic processing stations.

For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the drum surface may be described functionally, as follows:

A charge station A, at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic drum;

An exposure station E, at which a light or radiation pattern of a copy to be reproduced is projected onto the drum surface to dissipate the drum charge in the exposed areas thereof thereby forming a latent electrostatic image of the copy to be reproduced;

A development station C, at which the xerographic developing material including toner particles having an electrostatic charge opposite to that of the latent electrostatic image, are cascaded over the drum surface, whereby the toner particles adhere to the latent electrostatic image to form a xerographic powder image in configuration of the copy being reproduced;

A transfer station D, at which the xerographic powder image is electrostatically transferred from the drum sur face to a transfer or support material and then removed from the drum as by puffing; and,

A drum cleaning and discharge station E, at which the drum surface is brushed to remove residual toner particles remaining thereon after image transfer, and at which the drum surface is exposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.

It is believed that the foregoing description is sufficient for the purposes of this application to show the general operation of the xerographic reproducing apparatus. For further details concerning the specific instruction of the apparatus as shown, reference is made to the Rutkus patent and Osborne application referred to hereinabove.

As the drum shaped surface rotates past the transfer station A, a suitable backing material 12 such as paper is brought into contact with the xerographic surface 10 shaped as a drum by first sheet feeding mechanisms 14. The sheet and drum move at the same linear speed past a transfer corotron 16 which acts to electrostatically draw the toner image from the drum towards the paper. At the same time, however, the electrostatic charge emitted by the transfer corotron electrostatically tacks the paper to the drum. The drum and paper then travel together to the puffer 18 where jets of an aeriform fluid such as air separate the paper from the drum to allow the toner bearing paper to be transported by second sheet feeding mechanisms 20 to a fusing station for the production of the final copy. So far as has been described, the transfer and stripping operations are substantially the same as that disclosed in the aforementioned Rutkus patent and Osborne application.

In addition to the transfer corotron 16 and puffer 18, the instant invention further requires a detacking corotron 22, and a pre-transfer corotron 24 to effect its functions. The detacking corotron is positioned slightly down stream from the transfer corotron 16 so that an alternating current emission may be sprayed onto the back of the copy paper immediately after the electrostatic transfer. The purpose of this detacking corotron is to minimize the electrostatic attraction between the paper and drum as caused by the transfer corotron 16. The pre-transfer corotron 24 is also adapted to spray the drum with an alternating current but this is done preferably after development but before the drum-to-paper contact. The function of this pre-transfer corotron is to decrease the voltage range between the image and non-image areas of the latent electrostatic image so that such voltage variations on the drum have minimized effects in varying the rate of the corona emissions of the detacking corotron.

For the purposes of this illustrative example, let us assume that the original charge placed on the photoconductor is positive and that negative toner particles are used to develop the image. In such case a transfer corotron 16 with a positive DC output would attract the toner particles from the xerographic surface to the copy paper to cause the transfer of the image. Such positive emissions as caused by the transfer corotron 16 also tend to build up a uniform positive charge per unit area on the insulating copy paper. The transfer emissions accepted by the paper in regions overlying the non-image areas of the photoconductor induce an opposite or negative charge in the substrate portion of the xerographic drum in nonimage areas. This charge distribution in non-image areas of the drum and paper creates an electrostatic attraction therebetween.

The transfer corotron emissions thus tack the paper to the drum with a constant charge per unit area. The total charge between various copy sheets and xerographic surfaces vary, in proportion to the total amount of nonimage areas available which can accept charge. For instance, when copy has relatively small image areas, the background area is increased to thereby increase the total electrostatic attractive forces between the paper and drum. Conversely, when large solid area portions are being developed and transferred, there is a minimum of background area and as such the total amount of induced electrostatic attraction between paper and drum is proportionately minimized. As can be readily understood, a puffer to separate copy from a drum must be adapted to separate the highest possible total charge bond if it is to function properly. But when maximum puffing pressure is utilized, increased puffing smears occur, especially when a small total electrostatic bond is present.

The elimination or neutralization of the electrostatic bond between the paper and drum can be achieved by eliminating or neutralizing the forces creating the bond. Where the positive transfer emissions remaining in the paper create the bond, equal and opposite emissions can remove them.

The bond between the paper and drum consists of charges having a constant charge per unit area. It has been determined that the bond causing charge on the copy paper can be neutralized through the use of an AC corotron which produces an excess of emissions opposite in polarity of the charge of the paper. Such an AC emission can be achieved through the proper selection of the corotron wire and bias. The selected corotron with a successful detacking according to the instant invention includes a 3.5 mil platinum-iridium wire with a 4,500 RMS volt AC source at 400 c.p.s. with a positive 1,000 volt DC bias. The wire material and the diameter as well as the slight DC bias permit the production of the required excess of negative emissions. Although both positive and negative ions are emitted, the excess of negative ions will produce the required net negative effect.

The above described AC detacking corotron, the parameters of which are by way of illustration only, would in itself work to achieve the desired detacking function if no other variables were present. In practice, however, the charge distribution on the image areas of the photoconductor acts to vary the neutralizing negative emissions of the AC detacking corotron. The non-image areas of the xerographic surface are of a substantially lower potential than the image areas so that their effect is negligible in varying the negative emissions.

As the AC detacking corona source alternates between positive and negative, it exceeds the threshold potential in both directions to produce ion emissions, both positive and negative. The threshold voltage for a corotron is that voltage of a corona wire, beyond which, the air in the vicinity begins to break down for the production of ion emissions. When, however, extensive image areas and their associated charges exist on the xerographic surface portion presented to the detacking corotron, the positive emissions of the detacking corotron are suppressed by the like high potential charges on the xerographic surface. The decrease in positive emissions is accompanied by a proportionate increase in the negative emissions of the corotron. Conversely, when line copy with minimum image area charges are on the xerographic surface presented to the detacking corotron, there is little suppression of the positive detacking emissions. This is accompanied by a proportionate decrease of negative detacking emissions. Thus, because of the effect that varying charge distributions on the xerographic surface have on the AC detacking emissions, the negative or neutralizing emissions vary in their rate of discharge. The varying degree of neutralization by negative corona emissions is generally unsuitable in itself for detacking the charge bearing paper from drums due to the effects of varying extents of non-image areas.

To alleviate problems created by the non-uniform emission rate of the detacking corotron, the instant invention further incorporates a pre-transfer corotron to minimize the charge differential between image and non-image areas of the xerographic surface. By doing this, the entire area of the xerographic surface will suppress the detacking emissions in a substantially uniform fashion for stabilizing the rate of the detacking emissions.

Where, for example, an original charging potential is placed on a xerographic surface for an exposure which will bring the unexposed image area portions of the xerographic surface down to 1,000 volts and the exposed non-image areas down to 300 volts, there is a range of 700 volts on the xerographic surface. When such a potential range is passed adjacent the detacking corotron, it will detrimentally vary the rate of the detacking corona emissions. Theoretically speaking, complete AC detacking can be achieved only if the voltages across the xerographic surface are entirely uniform. In practice however, it has been found that by bringing the potential range on the xerographic surface to a decreased amount, a workable system may be employed to permit a substantially uniform rate of the detacking emissions required to practice the detacking according to the instant invention.

According to the present invention, it is contemplated that an adequate reduction in the potential differential between image and non-image areas may be achieved by exposing the image bearing surface to AC emissions with a net negative effect prior to the time when the paper contacts the drum. This is achieved by pre-transfer corotron 24. It has been found that where, for example, a latent electrostatic image bearing surface with a 1,000 volt build up on image areas and a 300 volt residual charge on non-image areas is exposed to suitable corona emissions, prior to transfer, the potentials on the xerographic surface may be reduced to 500 volts in the image areas and 250 volts in the non-image areas. As such, the differential between the potentials in these areas has been reduced from 700 to 250 volts. When such a reduced charge distribution differential on the xerographic surface is moved beneath the detacking corotron 22, with the paper electrostatically adhered thereto, such a minimized charge distribution has little effect in varying the rate of neutralizing emissions of the detacking corotron 22.

The reduction of image and non-image area voltages in different amounts occurs since a corona source acts to bring the acted upon charged surfaces to an equilibrium voltage with the corotron. In action, the corotron alters the areas whose potentials are farthest from the corotron potential as a function of time. Thus, as the developed xerographic surface passes beneath the pre-transfer corotron, the higher potential image areas become reduced in potential faster than the non-image areas. Although the corotron begins to reduce both image and non-image areas substantially simultaneously the faster voltage reduction rate of the image area charges for the time the xerographic surface is beneath the corotron has the net effect of minimizing the voltage differential between image and non-image areas.

A suitable pre-transfer corotron 24 for effecting such a reduction in potential on the xerographic surface may be achieved by an AC source'of 5,000 volts RMS at 60 cycles per second with a positive DC bias of 270 volts. A wire as used on the detacking corotron with the DC bias functions to permit an excess of negative emissions. The variable rate of negative emissions occurs as a function of the charge on the drum in the same manner as it does as a detacking corotron.

In operation, as the developed xerographic image on the drum-shaped surface 10 moves beyond the development station C, it passes beneath a first or pre-transfe'r corotron 24. At this point the potential on the image and non-image areas of the drum are both reduced and the potential differential therebetween is reduced. The image then passes to a point just before the transfer corotron 16 at which point a sheet of copy paper contacts the drum for concurrent movement therewith. Passage of the paper and image bearing xerographic surface beneath the second or transfer corotron 16 acts to electrostatically tack the paper to the drum and electrostatically transfer the toner image from the drum to the paper. Continued movement of the paper with the drum passed a third or detacking corotron 22 minimizes the electrostatic attraction between the paper and drum so that the jets of air produced by puffer 18 may be effective to separate the paper from the drum. Second sheet feeding conveyors 20 move the paper from the drum to the fusing station for the creation of final permanent copy.

When the pre-transfer corotron 24 and detacking corotron 22 are employed as described above, the puffer pressure required to separate the drum and paper may be reduced from 13 psi. to 5 psi. This reduced pressure minimizes the possibility of toner smear created by such a flow of air and further reduces the possibility of toner dust being blown throughout the other functioning elements of the machine.

It is obvious that the instrumentalities needed to carry out the instant invention are readily adapted for corporation into commercial xerographic machines.

While the present invention, as to its objects and advantages, has been described herein as carried out in a specific embodiment thereof, it is not desired to be limited thereby; but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is: 1. In apparatus for producing xerographic copy which includes means for charging a photoconductive surface with an electrostatic charge, means to expose the charged surface to dissipate the charge in a patterned configuration of image and non-image areas corresponding to the object to be reproduced, means to develop the exposed photoconductive surface with charged toner particles, means to electrostatically transfer the toner particles to an insulating backing sheet and means to strip the tonerbearing backing sheet from the photoconductive surface, the improvement comprising means to reduce the charge differential between the exposed and unexposed portions of the photoconductive surface prior to the electrostatic transfer and means to decrease the electrostatic bond between the backing sheet and photoconductive surface created by the electrostatic transfer to thereby facilitate the stripping of the toner-bearing backing sheet from the photoconductive surface.

2. The apparatus as set forth in claim 1 wherein the means to reduce the charge differential includes an AC corona emitting source adapted to produce emissions of a net polarity opposite from the polarity of the charge on the unexposed photoconductive surface.

37 The apparatus as set forth in claim 1 wherein the means to decrease the electrostatic bond includes a corona emitting source adapted to produce emissions having a net polarity opposite from the polarity induced in the backing sheet by the electrostatic transfer means.

4.- Apparatus for transferring xerographic toner images from developed photoconductive surfaces to insulating backing sheets and for stripping the toner-bearing backing sheets from the photoconductive surfaces including,

a first corotron positioned to subject a developed photoconductive surface to corona emissions having a net polarity opposite from the charge on the photoconductive surface to reduce the potential differential between portions of the surface,

means to bring an insulating backing sheet into contact with the photoconductive surface,

second corotron means positioned on the side of the insulating backing sheet remote from the photoconductive surface to subject the developed photoconductive surface and insulating backing sheet to corona emissions having a polarity opposite from the charge on the toner particles to thereby electrostatically attract the toner image to the backing sheet,

third corotron means positioned on the side of the insulating backing sheet remote from the photoconductive surface to subject the developed photoconductive surface and insulating backing sheet to corona emissions having a net polarity opposite from the emissions of the second corotron means to thereby minimize the electrostatic bond between the toner-bearing sheet and photoconductive surface caused by the second corotron means and means to strip the toner-bearing backing sheet from the photoconductive surface.

5. The apparatus as set forth in claim 4 wherein the means to strip the toner-bearing sheet from the photoconductive surface includes means to propel a jet of aeriform fluid between the toner-bearing sheet and photoconductive surface.

References Cited UNITED STATES PATENTS 2,959,153 11/1960 Hider 118-637 2,576,047 11/1951 Schaffert 118-637 XR 3,124,457 3/1964 Schwertz 118637 XR 3,244,083 4/1966 Gundlach 118637 XR 3,256,002 6/1966 Hudson 118-637 XR CHARLES A. WILLMUTH, Primary Examiner.

25 P. FELDMAN, Assistant Examiner. 

1. IN APPARATUS FOR PRODUCING XEROGRAPHIC COPY WHICH INCLUDES MEANS FOR CHARGING A PHOTOCONDUCTIVE SURFACE WITH AN ELECTROSTATIC CHARGE, MEANS TO EXPOSE THE CHARGE SURFACE TO DISSIPATE THE CHARGE IN A PATTERNED CONFIGURATION OF IMAGE AND NON-IMAGE AREA CORRESPONDING TO THE OBJECT TO BE REPRODUCED, MEANS TO DEVELOP THE EXPOSED PHOTOCONDUCTIVE SURFACE WITH CHARGE TONER PARTICLES, MEANS TO ELECTROSTATICALLY TRANSFER THE TONER PARTICLES TO AN INSULATING BACKING SHEET AND MEANS TO STRIP TONERBEARING BACKING SHEET FROM THE PHOTOCONDUCTIVE SURFACE, THE IMPROVEMENT COMPRISING MEANS TO REDUCE THE CHARGE DIFFERENTIAL BETWEEN THE EXPOSED AND UNEXPOSED PORTIONS OF THE PHOTOCONDUCTIVE SURFACE PRIOR TO THE ELECTROSTATIC TRANSFER AND MEANS TO DECREASE THE ELECTROSTATIC BOND BETWEEN THE BACKING SHEET AND PHOTOCONDUCTIVE SURFACE CREATED BY THE ELECTROSTASTIC TRANSFER TO THEREBY FACILITATE THE STRIPPING OF THE TONER-BEARING BACKING SHEET FROM THE PHOTOCONDUCTIVE SURFACE. 